The use of iron oxide in soda-lime glass systems for improved UV absorption is known. In such systems the iron oxide content has been up to 0.12 weight percent (wt. %). In these systems, however, this dopant level is known to reduce visible transmission of the glass, particularly in the 650-750 nm range, by 1-2% with glass thickness of 1 mm, an objectionable feature.
UV absorbing borosilicate glass compositions for electric arc discharge lamps such as high intensity discharge (HID) lamps, typically contain either lead and arsenic oxides or lead and cerium oxides. One conventional lead and arsenic containing borosilicate glass is SG772 which is used for both outer envelope and lamp stem applications. However, both lead and arsenic are toxic materials and it would be very advantageous to be able to manufacture acceptable glasses without using these materials. Arsenic oxide is generally employed in glass compositions as a fining agent for glasses which are difficult to fine (i.e., removal of bubbles). Ceria (&gt;0.15 wt. %) has been used as an acceptable substitute for arsenic oxide for fining glasses, and providing UV absorption. However, ceria-containing borosilicate glasses will solarize under UV irradiation when lead oxide is not in the composition. Solarization produces light absorbing color centers that darken the outer envelope and seriously reduce light output. Elimination of the lead and arsenic oxides has produced workable glasses with adequate sealing capabilities to tungsten; however, such glasses (for example, Schott 8487, which is used for lamp stems and tubulations) do not absorb sufficiently in the UV region to be used as outer envelopes. Additionally, Schott 8487 contains a high concentration of B.sub.2 O.sub.3 (16.9 wt. %), which makes for a more volatile glass composition, and the potential for surface scum formation when melted in a conventional, gas fired furnace. Surface scum is a highly siliceous glass which forms when the glass volatilizes some of its constituents to form a new, unstable glass composition at or near the surface of the melt. This unstable composition is prone to devitrification, phase separation and low refractive index optical cord. To prevent glass defects, a highly volatile glass is commonly melted in an electric, cold crown furnace which inhibits surface volatilization, and surface scum from forming. Electric melting is however a very expensive solution. Schott 8486 bulb glass contains far less B.sub.2 O.sub.3 (12 wt. %) and as such is less prone to scum formation, however it does not seal to tungsten wire and does not have sufficient UVB absorption to be used as an outer envelope for HID lamps. As an illustration, the Schott glass enumerated above has a transmittance of 17% at 300 nm, whereas open fixtured lamps employed in the United States must meet the requirements of UL1572 Specification which requires a transmittance of no more than 8% at 300 nm. Additionally, to be an acceptable glass for use as the outer envelopes of electric arc discharge lamps, any absorption at wavelengths approaching the visible, say, above about 375 nm, must be minimized.
A lead and arsenic free borosilicate glass is described in U.S. Pat. No. 5,557,171 to Marlor et al. which is incorporated herein by reference. While this glass is acceptable for use in the outer envelopes of high intensity discharge lamps, it is difficult to produce in large quantities. In particular, in conventional gas fired glass melters, excess batch quantities of iron oxide must be added to achieve the desired level of UV absorption because of the tendency of the UV absorbing Fe.sup.3+ species to be reduced to Fe.sup.2+. The higher batch amount of iron oxide leads to an increase in the presence of Fe.sup.2+ which produces a bluish-green coloration and a reduction in the visible transmission of the glass. Additionally, the glass tends to form surface scum on the melt surface which causes high levels of glass defects such as optical cords, knots and stones.